Mastitis affects the health and welfare of dairy cows worldwide. Coagulase-negative staphylococci (CNS) are known to form biofilms and are increasingly recognized as a cause of persistent bovine intramammary infections. A total of 90 CNS isolated from cows with clinical and subclinical mastitis in Argentina from 2008 to 2014 were identified by PCR-RFLP using the gap gene. Standard microtiter plate assays were used to assess CNS biofilm formation, and Staphylococcus haemolyticus species formed the strongest biofilms. The presence of biofilm-associated genes icaA, bap and aap was detected in a few isolates, while embP, fbe, atlE and eno were present in the majority of isolates. Genes encoding resistance to β-lactams were detected among the isolates; blaZ, mecA and mecC were detected in 21, 4 and 1 isolate, respectively. Resistance to macrolides and lincosamides (n = 6) was attributable to ermB, ermC, mphC or mrsA or a combination of those genes. In this study, we identified CNS species involved in mastitis and provide information about pathogenicity and antimicrobial resistance, which is essential to design efficient strategies to control mastitis caused by CNS.

Mastitis is a disease that affects the health of cattle and causes great economic losses. The most 24 prevalent microorganisms in Argentina belong to the genus Staphylococcus, being the main pathogen 25 S. aureus, though coagulase-negative Staphylococci (CNS) have gained importance for the ability to 26 express virulence factors and antimicrobial resistance. The aim of this study was to determine 27 phenotypically in vitro minimum inhibitory concentrations of betalactams and macrolide-lincosamide 28 against staphylococci isolated from mastitic milk. We studied (n=162) isolates, (n=65) S. aureus and 29 (n=97) CNS. The 33,8% (22/65) of S. aureus presented resistance to penicillin, being the MIC90: 0,5 30 µg/ml, the 7,7% (5/65) to erythromycin with a MIC90: 1 µg/ml and 6,1% (4/65) to clindamycin with a 31 MIC90: 0,1 µg/ml. The 30,9% (30/97) of CNS were resistant to penicillin being the MIC90: 0,25 µg/ml, 32 the 7,2% (7/97) to erythromycin with a MIC90: 0,5 µg/ml, the 4,2% (4/97) to clindamycin with a 33 MIC90: 0,12 µg/ml and 3,1% (3/97) to oxacillin being the MIC90: 0,38 µg/ml. Methicillin-resistance 34 was not observed in S. aureus. Bacteriological diagnosis and susceptibility determination with proper 35 management of dairy herd are essential for disease control.